Countdown to Hekla

It has been a long time since I wrote about Hekla. But, I guess nobody is surprised at what I am about to write.

Everyone with a genuine interest in volcanoes have their favorite volcano. As many in here know Hekla is my favorite volcano to bang my head against. Few volcanoes are as intricate as Hekla, and few have such a short run up before an eruption as Hekla. Normal run up time to an eruption is between 30 – 80 minutes from the first sign.

This time around has been different. But let us first recapitulate what has happened since the last eruption. For those who are curious about how Hekla works I would like to recommend my own post about her innards:

http://volcanocafe.wordpress.com/2012/01/10/deconstructing-hekla-hells-gate-revisited/

Background

In 2004 Hekla had received as much new magma as was discharged during the 2000 eruption and sometime during late 2008 to early 2009 that figure had doubled. After that the inflation stagnated and no real uplift was measured at the GPS-stations with the exception of what was most likely magma moving between the different magma chambers.

During the summer of 2011 earthquakes was registered and a public safety alert was issued stating that Hekla was close to erupting. From then on Hekla has had earthquakes ranging from miniscule to 2M+ without erupting. For those who are not familiar with Hekla one should notice that she normally is aseismic, or in other words, that she does not have a lot of earthquakes.

From 2010 and onwards Hekla started to show a new feature that I dubbed “transients”. The transients are sudden rapid drops in the strain measured at the borehole strainmeters. These transients have only been seen before as Hekla erupted. They had before 2010 never been seen without an eruption occurring. A transient is in short happening as the mountain strains to open up.

On the 13^th of March and onwards Hekla had a swarm of earthquakes and once again transients were noticed on the strain-meters. There was also harmonic tremor measured indicating rapid movement of magma. This caused IMO to issue a public safety warning, and the London VAAC issued a flight code warning Orange. This followed the exact pattern of how all the previously instrumentally monitored eruptions had started so far.

As we all know nothing happened in the end. We can now safely say that we know even less than we did before about how Hekla acts before an eruption. Because now we have to figure out why Hekla did not erupt when she should have. I guess someone will have a research career out of it in the end.

Present

After this Hekla entered into a new phase never seen before, this time a phase of very rapid and unbroken inflation started. What happened is most likely that the earthquake swarm removed blockages inside the deep feeder tubes of Hekla enabling fresh magma to flow into the volcanic system.

The rate of inflation varies a lot depending on where the GPS station is placed. The big exception is Mjóaskard situated to the west of Hekla. It has only suffered an uplift of 5mm in the last 5 weeks. For the other stations the rate of inflation is between 15mm in Hestáalda and 32mm at ISAK. Average uplift is 16mm, and 21mm if MJSK is not counted. This type of rapid inflation has so far never been measured at Hekla.

http://strokkur.raunvis.hi.is/~sigrun/HEKLA.html

During the entire inflation phase there have been scattered earthquakes and micro-quakes.

If the current rapid inflation continues there is a very low chance of Hekla not erupting. Yes, we do not know what is happening with Hekla now since we have never seen this type of behavior. But Hekla is constructed in such a way that she can’t take a huge increase in pressure without erupting.

Image by GeoLurking based on data by the University of Iceland and Professor Sigrún Hréinsdóttir. All areas are showing uplift on this image covering the period from 4th of April up untill now. The area with the highest uplift are due north of Hekla.

If the inflation continues at the current rate Hekla will erupt. When? Well I am not going to make any bets, but any time from 1 hour from when you read this to 4 weeks. Remember that 4 weeks into the future the combined uplift in 2 months will have exceeded 50mm at many GPS stations. As seen on the image above the largest uplift is happening on the northern slopes. This is a known site for one of Heklas primary magma chambers. The area to the northeast are not showing correctly, there is uplift there too, but due to lack of a GPS station there the model get scewed.

Image courtesy of the University of Reykjavik and Professor Sigrún Hréinsdóttir.

I personally would not at any cost get closer to Hekla then 10 km from now on. And then I would stay in the car on the road. If you are closer the chance of you surviving is not good and 5 km the chance of you surviving the initial blast is pretty much nill.

What will the eruption be like? Here I will be guessing since Hekla has changed her behavior compared to the last eruptions. I would say that Hekla has remobilized old evolved magma during all that moving of magma, and this latest inflation phase seems to fill up a lot of old magma chambers. This causes me to fear a rather explosive start of the eruption. I would also say that there is quite a high likelihood of there being more lava erupted then was seen during the last 3 eruptions. I will hedge my bet by saying that I would expect it to be anything between a VEI2 and a VEI4 on the volcanic explosivity index, and that Hekla will effuse between 0.1 to 2 cubic kilometers of lava. Based on the GPS plot above my best judgement is that the eruption will start at the top of Hekla proper and then open up the fissure both to the south, but mainly to the north. Most probably the Hekla fissure will open over all of Hekla proper with a fissure extending to the Northeast.

For those who wish to follow the eruption, here is the Hekluvöktun page:

http://hraun.vedur.is/ja/hekla/

CARL

Bárdarbunga calling

Image Wikimedia Commons

Á SPRENGISANDI

Ríðum, ríðum, rekum yfir sandinn,

rennur sól á bak við Arnarfell.

Hér á reiki’ er margur óhreinn andinn

úr því fer að skyggja á jökulsvell.

.

.

.

This is one of the most famous Icelandic folk songs, talking about people riding over the highland desert, on the Sprengisandur track, hurrying on horseback over the barren land between the majestic glacier volcanoes, thinking about ghosts haunting the wind-blown valleys between Hofsjökull and Vatnajökull.

This is Hofsjökull glacier volcano. But they will also be traveling past Bárdarbunga volcano. Without being as well-known and popular with the media  as her colleagues Katla and Hekla, Bardarbunga is a volcano which deserves remembering.

Bárdarbunga, a big volcanic edifice and central volcano located under Vatnajökull in Iceland, has been showing unrest for some time now. There was esp. the glacier run (jökulhlaup) from June 2011 into the reservoir Hágöngulón in the highlands of Iceland. But also lately, growing unrest showed on the tremor charts of IMO:

Under the five big ice caps which together cover about 10 % Iceland’s surface, there are located 9 or 10 central volcanoes incl. parts of their fissure systems. Under Vatnajökull alone, there are 2 twin systems of basaltic and rhyolitic central volcanoes, i.e. Grímsvötn-Gjálp and Bárdarbunga – Hamarinn/Loki-Fögrufjöll,  Kverkfjöll in the north, all of these part of the EVZ (Eastern Volcanic Zone of Iceland), and two separate active central volcanoes, Esjufjöll and Öraefajökull, forming a separate belt in the east. (Thordarson, 2007)

Bárdarbunga central volcano, proudly presenting a 80 km2 caldera with a depth of 700 m, and reaching heights of  2100 m at its summit, is placed in the northwestern part of Vatnajökull ice cap, north of Grímsvötn.

Of these volcanic systems, Grímsvötn is by far the most productive with 480 eruptions in 6.500 years, whereas Bárdarbunga prehistoric eruption frequency (which means in Iceland till 1.000 BP) reaches 330 eruptions during the same period of time and an average of 5 per century. On the other hand, tephra layers from Bárdarbunga are dispersed more widely, which indicates „more voluminous and/or longer lasting eruptions than these of Grímsvötn“. Additionally, there were 135 tephra layers which could be identified with Grímsvötn, 87 originating from Bárdarbunga and only 17 from Kverkfjöll since Icelandic settlement started in the 9^th century. This gives Bárdarbunga the place of the 2^nd or third most active volcanic system in Iceland in historic and prehistoric times – rivaling Hekla. (Larsen,  2011). But probably there were much more eruptions, mostly very small ones which couldn’t break the glacier surface. See also GVP:  http://www.volcano.si.edu/world/volcano.cfm?vnum=1703-03=&volpage=erupt

But Bárdarbunga also has the longest or perhaps – if Askja is competitive enough – the second longest fissure system of Iceland (around 190 km), extending to the south into the Landmannalaugar region (Veiðivötn, Vatnaöldur) and also some 50 km in northeastern direction from her outlet glacier Dyngjujökull. It produced not only one of the most important flood eruption events in historic time (Veiðivötn in 1480), but also an enormous lava field, the so-called Þjórsárhraun (2130 km3) 8.500 years ago.

There has now been some earthquakes, one of them magn. 3.3. Like many other calderas, she shows unrest from time to time, which may or may not mean more. This is anyways a volcano which deserves some attention.

See also Carl’s post about Hamarinn: http://volcanocafe.wordpress.com/2012/01/25/hamarinn-volcano-burns-night/

Literature:

• Volcanism in Iceland, by Th. Thordarson, G. Larsen (2007)
• Basaltic Tephra Layers reveal the Eruption History of the Icelandic subglacial Volcanoes, Grimvötn, Bardarbunga and Kverkfjöll during the last 7000 Years, by B. A. Oladottir, G. Larsen, etal. (2011)

PS: Disclaimer – I am just a layman interested in geology.

Inge B.

(Images Wikimedia commons)

Name that Volcano Riddle!

Resident of Siglufjördur looking rather wooden eyed after yet another sleepless night with earthquakes.

The week has been calm eruptionwise, but fairly frisky earthquakewise in Iceland. The transform faultline north of Siglufjördur kicked into high gear with continous medium sized earthquake swarms.

The active faultline is one of two major faults making out two of the boundaries for a microplate that is seismically locked at its southeastern corner under Theistareykjarbunga volcano. Theistarykjarbunga is as most readers of this blog know one of the two volcanoes in Iceland and the world competing for the title of having had the largest flood basalt eruption during the last 10 000 years, the other one being Bárdarbungas Thjórsahraun eruption.

Image by IMO. The earthquake band of the fifth most powerfull earthquake swarm during the last 12 years.

The earthquake swarm has so far had a couple of hundred earthquakes in them, 13 between 3 to 4, and 5 earthquakes from 4 and upwards to 4.3M. This is a fairly vigorous, but not unheard of amount of energy released from an Icelandic earthquake swarm. It is though the 5th largest during the last 12 years. There is currently no other signs that this will lead into anything too exciting in the near future.

RIDDLE

This week we will try a new version of the friday volcanic riddle game. It is a “Name that Volcano Riddle!” by commenter Suzie. One point to be had.

The French footie fans looked on in horror as their European opposites ran riot round the capital! They asked themselves “What does this unruly orange mob mean to us?”

VOLCANOSPORTS

During the week I tried to come up with some extremely extreme sports that you can do if you have a volcano handy. My favourite was a surfing down a Hawaiian lava stream on a ceramic surfboard. The idea though was not as novel as I thought. Apparantly it is big too skate down scoria cones.

TGIF!
CARL

Odd strain at Hekla

Photograph by Eggert Norddahl, used under exclusive permission. All rights reserved. To use this or any other image by Eggert, contact Volcano Cafe or Eggert Norddahl directly.

Hekla have one of her days today. It all started at 08.56 with a small earthquake located at Árnes close to Haukadalur SIL-station. The earthquake was clearly tectonic and at a respectable depth of 6.2km. The earthquake will most likely be revised later as it is of low probability due to low energy released. What makes this earthquake interesting in relation to what followed is that during the weeks before Hekla 2000 the area had a small number of low energy earthquakes.

Image by Icelandic Met Office (IMO). Location of the very small earthquake that started todays festivities.

A couple of minutes later the new (installed after the 2000 eruption) borehole strainmeter at Hekla registered a medium sized, but long lasting negative mountain strain transient. During the last eruption the largest recorded strain was recorded at the more distant Búrfell. The idea of installing Hekla borehole strainmeter was to see if it is possible to pick up pre-cursor strain falls. As I am writing this the negative drop in strain is still ongoing.

Image by IMO. The initial stages of the mountain strain recorded by the Hekla Borehole strainmeter.

A couple of small spikes are showing on the Mjóaskard SIL-station, but they are to small to pinpoint them as earthquakes, at least the exact location. Also, a small tremor ranging from 1,5Hz and upwards started at 11.35 local time.

Image by IMO. Strain at -6E+04 and counting. Negatve mountain strain building up.

Is this then the run up for an eruption? Sofar it is not likely. We still miss the tell tale swarm of small earthquakes (0.2 to 2.0M), a large negative strain drop at Búrfell borehole strainmeter, and of course a rapid increase in harmonic tremor on all stations. This can though change at any time. As mountain strain builds up the likelyhood of earthquakes increase over time.

I would not recommend climbing Hekla today.

CARL

What’s going on at Katla? Part III

Image from Wikimedia. Aerial picture of Katla.

Trying to make sense of complex phenomenae

In the first two instalments, we had a look at Katla as she appears through media and what she has done historically. It is now time to have a look at what’s going on and try to paint a coherent picture of what she actually is, is up to and able to do, but first let us recapitulate what we found previously:

• There is a general interest in Katla because she is and has been regarded as a very dangerous volcano by generations of Icelanders.
• The presentation of Katla in media is skewered by vested interests ranging from scientists who hope to increase their professional and/or public standing, people trying to cash in on the interest generated such as journalists and bloggers, and finally, there are people trying to increase their standing within the subculture of doomsaying and alarmism.
• Katla is a massive but relatively young volcano, located on the MAR, and formed when Iceland was covered by glaciers.
• The records include two large fissure eruptions on the NE flank of Katla; the prehistoric 5 km3 Hólmsá Fires of 5550 BC and ~22 km3 Eldgjá eruption in 934 AD. In historic times, the 1100 years or so that Iceland has been settled, there have been 27 listed eruptions (28 if the inferred minor subglacial 2011 eruption is included), 23 of which have been explosive.
• Of the 23 explosive eruptions, three have been assigned VEI 3, thirteen VEI 4 and four VEI 5.
• The four VEI 5 eruptions are remarkably alike in size at 1.2 – 1.5 km3, which is at the upper end of what Katla probably is able to do but at the very lower end of VEI 5 eruptions.
• Tephrochronology (in some cases complemented by radiocarbon dating) has identified a further 103 eruptions going back ~8,500 years, and in the few cases where a VEI has been assigned, none have been greater than a VEI 4.
• Katla does not possess a caldera-sized magma chamber.
• In order to account for the great number of explosive eruptions which involve more evolved magmas, Katla could have more than a single magma chamber.
• The available evidence suggests that in order to break through the up to 700 meters thick Mýrdalsjökull glacier, an eruption must be at least a substantial VEI 3.
• Direct and (primarily) indirect evidence suggests that smaller eruptions, mainly basaltic VEI 0 – 2 eruptions are severely underrepresented in her eruptive record and ought to exceed the number of observed eruptions.

Fig 1. Mýrdalsjökull showing the main glacier outlets, directions of jökulhlaups and areas affected. E –
Entajökull, S – Sólheimajökull, K – Kötlujökull, M – Markarfljot, Ss – Sólheimasandur, MS – Mýrdalssandur.
Eyjafjallajökull is to the left and the smaller glacier above is Tindfjallajökull (adapted from Google Maps).

The greatest danger from Katla comes from the very quick and extensive melting of the glacier caused by large eruptions which results in destructive jökulhlaups. There are three major outlets from the glacier: Entujökull to the NW that empties into the Markarfljot river and valley north of Eyjafjallajökull, Sólheimajökull to the SSW that empties onto the Sólheimasandur and finally, Kötlujökull to the SE that empties in a great arc east through south onto the Mýrdalssandur. What ought to be prime farmland and in fact once was settled, is nowadays an unsettled wasteland because of the devastating jökulhlaups unleashed by Katla. This is the true reason why Katla is considered to be such a dangerous volcano.

The fact that one often comes across the reference that in the days before the Hringvegur (ring road), “people were afraid to traverse the Sólheima- and Mýrdalssandur because of the frequent jökulhlaups” is another indication that smaller and unrecorded eruptions that cause only minor hlaups are far more frequent than the 40 – 80 years often given as the interval between main, and thus visible, eruptions.

Fig. 2. The foundations of the old bridge across the Múlakvísl river destroyed by the July 9th 2011 jökulhlaup
are visible to the left. The new bridge was laid down a week later. (photo John A Stevenson, GVP website)

Apart from the postulated connection between the Eyjafjallajökull and Katla volcanoes, one question that always crops up is the Goðabunga cryptodome. Many volcanologists maintain that it is a part of the volcanic system of the Katla central volcano. Others, notably Sturkell and his co-workers, claim it is part of the Eyjafjallajökull volcanic system. In order to shed some light on this issue, I asked our own GeoLurking if he could make a plot of all the earthquakes from 1994 up to and including the 2010 Eyjafjallajökull eruption. The results are quite clear and do throw up a surprise:

Fig 3. E-W cross section, view from south, through Eyjafjallajökull, Goðabunga and Katla. Plot by and
courtesy of GeoLurking. The “lines” formed at approximately 5, 3 and 1.1 km at Goðabunga and Katla are most
likely artefacts caused by quakes being assigned a poorly defined depth. The latter, 1.1 km, is the default depth
assigned by the automatic system in case it cannot compute a depth within the predetermined level of certainty and unless they are manually checked, which is not the case of every quake, automatic depth remains uncorrected, hence these artefacts.

From this cross section, it is quite clear that there is no connection between the Eyjafjallajökull volcanic system and Katla. Eyjafjallajökull has its own, well-defined feeder system from the Moho (first molten layer beneath the Earth’s solid crust) as does Katla, thus they are wholly independent of one another. As can also be seen, albeit not as clearly, Goðabunga too seems to be independent of either Eyjafjallajökull and Katla, the ramifications of which will be the subject of a later post by Carl. Sufficient to say that when we contemplate what Katla herself may be up to, we must differentiate between activity at Goðabunga and activity at Katla. Once we do, we see that while Goðabunga is more or less continuously active, Katla operates in bursts and seems to be most active during summer and autumn when the ice cap is at its, relatively speaking of an up to 700 m thick glacier, thinnest.

Fig 4. Activity post-Eyjafjallajökull. Activity at Eyjafjallajökull is minor and has to do with the system settling down after the end of the eruptive phase. Note that at a depth of 0 to 5 km or so, there seem to be three separate areas of activity at Katla. (Plot by and courtesy of GeoLurking.)

After the Eyjafjallajökull eruption, Katla seems to have entered an active phase with a suspected subglacial eruption on July 9th 2011 and several pits or craters forming on top of the glacier. This activity seems to be localised to three main areas within the caldera:

Fig. 5. Earthquake activity at Katla July 9th 2011, the day of the jökulhlaup and suspected subglacial eruption. Both the 1823 and 1918 eruptions occurred close to but just east of this area. The 1823 eruption occurred close to the easternmost red spot while the 1918 eruption was roughly at the rightmost dark blue spot below it. (IMO)

Fig 6. Earthquake activity at Katla July 17th 2011. (IMO)

Fig. 7. Earthquake activity at Katla July 21st 2011. The 1755 eruption was situated in the same area as the three overlapping orange spots. (IMO)

As can be seen, there are at least three distinct areas of activity inside the caldera with the one associated with the inferred July 9th eruption well to the south. The pits formed in the glacier also align with these three areas, albeit the pits to the northeast seem more drawn out along the caldera wall and not over the center of activity. These three areas seem to tie in with the three areas of activity noted in fig 4 as do the locations of three of Katla’s major eruptions. Thus there is not a single vent, but at least three at surface distances of approximately 5 to 8 km from each other. It is equally likely to judge from Fig 3. and Fig 4. in conjunction, that at great depth, they do have a common source.

I will now present you with my personal view of Katla, but do not be afraid to disagree or draw your own conclusions (within reason please, no Katlatubos here):

Katla is a young volcano and far more active than has previously been thought. Unlike the similarly aged but much less active Eyjafjallajökull, Katla has had more time to develop her system of sills to the point where they are fewer in number than they originally were but have a substantially larger magma-carrying capacity and approach or may have reached the point where they can be considered magma chambers proper. Since cooking evolved magmas takes a long time, usually millennia in the case of cubic kilometre-sized silica-rich magmas and at the very least many centuries for intermediate magmas, it is highly likely that Katla possesses several pockets of magma capable of eruptions ranging from high VEI 3s to small VEI 5s. Not only do the times between such eruptions argue this, their wide spread of location within the caldera does so too.

The most common type of eruption at Katla is the small, subglacial eruption of a few tens of millions of cubic meters of basaltic magmas. These eruptions are not energetic enough to break through the very thick Mýrdalsjökull glacier and the only proofs of their existence are intense earthquake swarms followed by minor jökulhlaups and later observations of deep pits or craters, sometimes water-filled, in the glacier ice. My guesstimate is that there may be many such small eruptions over any given ten-year period, and possibly in the case of a period of high activity, there may even be more than one in a single year. By back-tracking and investigating old accounts over the past few centuries of jökulhlaups in the area not associated with visible eruptions, it ought to be possible to identify many of these minor eruptions.

While a larger “proper” eruption of Katla in the VEI 3 – 5 range cannot be ruled out, I find one unlikely at present as the current activity mostly is in areas already depleted of evolved magmas by geologically speaking very recent major eruptions. Also there is little sign of the uplift required on GPS. If one were to occur, the odds for one towards the upper end of what Katla is able of ought to be better in the Eastern to Northern parts of the caldera.

Finally, what we do see when we look at SIL-stations such as Austmannsbunga, located on the NE caldera rim (not a coincidence, see above), is hydrothermal activity following a period of possibly still ongoing magmatic intrusion and not signs of an imminent, large eruption.

Fig 8. Hydrothermal activity at Katla as shown on the Austmannsbunga SIL (IMO)

I’m sorry to be such a boring old fart, but if this is unsatisfactory, start looking for intense earthquake activity at some 25 – 10 km depth, showing on the IMO map for Mýrdalsjökull as being in the Eastern to Northern part of the caldera, activity that shows a clear upwards trend and spreads when it reaches depths approaching 5 km!

HENRIK